Page 233 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 211

aggregates that move along the bottom section of the trained into the system. Dense phase transporta-
pipe, similar to movement of sand dunes in a desert, tion eliminates these problems that are commonly
with a dilute phase layer of solids moving above the associated with dilute phase conveying.
dunes. & Smaller conveying pipeline sizes due to heavier line
& The main principle of a dense phase conveying loading capabilities.
system is to slow down the velocity of the product & Smaller dust collection requirements at the material
in the pipe to a point that is below the speed at which
destination due to lower conveying air volumes.
the product breaks or degrades. In a dense phase
. What are the disadvantages of dense phase conveying
system, the velocity range at the source can be as low
systems?
as 5–8 km/h for the majority of products. The product
& Dense phase systems are more complicated than
velocity at the destination is always a function of the
dilute phase systems to control properly. The narrow
system DP.
operating range for stable operation gives rise to
& Because of the compressibility of the air or gas used
many control problems. It is necessary to constantly
as the conveying medium, the gas will expand as it
change the gas amounts for varying conditions in the
moves to the end of the pipeline and the product
system. The gas control device must be flexible and
velocity will increase accordingly. The terminal ve-
accurate.
locities of a dense phase system can vary, but in most
➢ Varying product transfer capacities, varying pro-
cases they rarely exceed 30 km/h. In a typical dilute
ducts, or varying product grades that result in
phase system, the starting velocities begin at about
dissimilar physical product characteristics pose
65 km/h while the terminal velocities can reach
problems in operation. Even product temperature
160 km/h.
variations need to be known at the design stage and
. What are the advantages of dense phase conveying over
incorporated into the system design.
dilute phase conveying? ➢ As temperature changes, velocity changes, in ad-
Dense Phase Conveying
dition to product characteristics.
& Decreased energy usage due to significantly reduced
. What are the different systems that are used in dense
volumes of air.
phase conveying?
& Reduced material breakage or degradation and re-
& Gatty System: Air is injected from a subpipe
duced pipeline wear due to lower conveying veloc-
mounted inside the conveying line. Used for convey-
ities. Best suited for handling friable products.
ing plastic pellets.
➢ Many products within the food industry fall into
this category. As an example, if a consumer opens & Buhler Fluidstat System: A series of bypass lines are
a bag of cheese puffs and finds that half of them are used every half a meter or so. Solids having good air
broken into little pieces, he/she will quickly retention properties are well handled by this system.
change product brand to that with whole cheese & Tracer Air System: Uses a series of boosters employ-
puffs. Degradation of the final product is a very ing a pressure regulator and a check valve.
important factor for any manufacturer of goods. & Pulsed Conveying System: Consists of a flow vessel
& Advantageous for handling abrasive solids as in and an air knife downstream from the discharge
dilute phase conveying velocities are higher resulting valve. The plug lengths are controlled by the air
in increased wear of pipe and associated elements. knife operation time.
Many minerals and chemicals fall into this category. & Takt-Schub System: Incorporates a double pulse
The pipe wear can also result in a contamination system employing two gas inlet valves whose timing
problem because the pipe material will get mixed can be controlled. The system is operated in an
with the product. alternating pulsing manner.
& Another advantage of using the dense phase system & Plug Pulse System: The dischargevalve is alternately
for plastics transport is the following: operated with a gas pulse. It works well with free-
➢ Some of the softer plastics, such as polypropylene flowing materials.
and polyethylene, smear onto the pipe wall when & Molerus–Siebenhaar System: This system relies on
the product slides along the outer wall of an elbow vibrations being put into the system by an unbal-
in a dilute phase transport system. The plastics anced motor. These vibrations cause the materials
actually melt due to the frictional contact with the to move with a low velocity throughout the system.
pipe wall and leave a long thin layer of material. This system is particularly useful for fragile
The layers are peeled off into strips and reen- materials.